Wind Drag & Turbulance

[Kankujoe]

I towed my TTT about 600 miles this past weekend. Since the price of fuel seems to be constantly on the rise I've become much more interested in towing efficiently and trimming fuel costs. During my recent trip I noticed a 15%-20% drop in gas mileage while towing my TTT. Doing some quick calculations I decided to try a "Wind Deflector" on the top of my vehicle in an attempt to improve fuel economy. My tow vehicle has sufficient power to easily (but maybe not economically) tow a trailer 5 times as heavy or several times larger. Aside from my gas mileage and the occasional steep grade the TTT was barely noticeable. Most of my towing was in overdrive gear using cruise control.

I've been researching online the effects of Wind Drag & Rear Turbulence on fuel economy. When my trailer was manufactured (1992) no one was concerned about aerodynamics and therefore it is quite "Square." It is also slightly over a foot taller and about a foot wider at the top than my tow vehicle... it is definately not aerodynamic (a big plus for those with some of traditional TearDrop designs). One interesting point I learned online is that the 'drag' turbulence/vacuum from the rear is a major factor (my TTT is quite Sguare there as well).

I bought a light weight wind deflector 48" x 15" for $150 to try on my next outing. The promo ad description on the deflector claimed that it should improve gas mileage up to 15%.

I'd be interested in the experiences of others in the towing & fuel economy and anyone's experience with using a 'wind deflector.'

[asianflava]

This is what I learned last week while towing my tear with "The Mighty Tundra" as it has been named.

According to the in dash computer:
75mph = 13.8MPG
70mph = 16.4MPG

Needless to say, I kept it at 70 on the way home.

[Kankujoe]

This is what I learned last week while towing my tear with "The Mighty Tundra" as it has been named.

According to the in dash computer:
75mph = 13.8MPG
70mph = 16.4MPG

Needless to say, I kept it at 70 on the way home.

What is typical when not towing your tear?

[asianflava]

Don't know, as soon as I hit 500miles I hitched up the trailer. that 500miles was mostly city driving. Probably still some break-in associated with the bad gas mileage also.

[angib]

I seem to repost this every 6 months or so, so apologies to those who've seen it before.

VW did test a roof deflector in their wind tunnel about 20 years ago, which I think is one of the few claims about these deflectors that I trust. They used an early VW Passat and a Yurpeen-sized trailer (ie, much larger than the tow vehicle):



From the first case to the second, with the roof spoiler, the drag drops by 15%, so that would exactly match the mileage claims you've seen.

Incidentally, the last case with the sloped front (now the standard design for trailers in Yurp) has exactly the same drag as the first case - not lower as some people might think. However the sloped-front trailer doesn't suffer from any reduction in hitch load at speed, which the square-front trailer does - it loses about 2/3rds of its hitch load by 50mph, which affects its stability.

Andrew

[Kankujoe]

Andrew,

Thanks for the post & diagram...

Any chance you having a reference to the VW study so that I could find it and read it?

Did the study address the rear turbulence/drag or the vortex that is formed to the rear of the trailer?

[angib]

Any chance you having a reference to the VW study so that I could find it and read it?

Pesche W, Mankau H, 'Pendel-Zug, Auftriebskräfte am Wohnanhänger beeinflussen die Stabilität von Wohnwagengespannen, Automobil-Revue 18, Berne, 1982, pp51-53.

Well..... you did ask!

I've taken this graph from 3 or 4 about trailers in the 'bible' of aerodynamics - Aerodynamics of Road Vehicles, by Hucho. This is now published by SAE and costs around $100 - but maybe your local library can get a copy? It's now up to a 4th edition, so there may be more stuff or more modern stuff than in my first edition.

There's nothing about flow behind a trailer, but elsewhere in the Hucho book there's stuff about the flow behind truck bodies which would be fairly similar.

Andrew

[swizz]

Hi Andrew,

just read this post & found it of interest - I haven't seen one of those deflectors for ages!

Re: the square fronted trailer causing reduced noseweight at speed - when using the deflector, was there any detail on what effect the deflector had in this respect?

I suppose you'd expect it to provide its own load on the rear of the vehicle at speed - just curious thats all - these German car designers are good at making their spoilers pop up at speed automatically aren't they?

[Kankujoe]

Used my new "wind deflector" this weekend. Noticed a little over 10% increase in gas mileage. Not really certain as my trip route was quite a bit different the week. I was hoping for more. About half of my trip this weekend (550 miles) was highway, the rest was curvy and up & down country roads.

I need to play around with the pitch angles on the deflector, it is adjustable from zero to about sixty degrees. Also interested in possibly installing some sort of rear fins to deflect the drag to lessen the vortex effect.

[bobhenry]

Used my new "wind deflector" this weekend. Noticed a little over 10% increase in gas mileage. Not really certain as my trip route was quite a bit different the week. I was hoping for more. About half of my trip this weekend (550 miles) was highway, the rest was curvy and up & down country roads.

I need to play around with the pitch angles on the deflector, it is adjustable from zero to about sixty degrees. Also interested in possibly installing some sort of rear fins to deflect the drag to lessen the vortex effect.

http://www.rexresearch.com/patterson/patterson.htm
I found this real curious !! This whole web site is quite interesting !

[Arne]

There was a lot to read at the link, but the results seem way too good to be true.

First I don't see how it could affect the bottom or the sides.. second, just pushing the front through the air would take a lot of doing.

But, I'm willing to be convinced if someone can do it for me.

[Kankujoe]

http://www.rexresearch.com/patterson/patterson.htm
I found this real curious !! This whole web site is quite interesting !

Very interesting to read but I don't have enough physics/engineering background to tell if it is legitimate or if it is BS...

[Lesbest]

Check out airtabs, for the back of trailers. They are vortex generators and supposed to smooth out the airflow at the rear of square trailers. Might be worth a try.

Airtabs.com

Hope they help.
Les

[mikeschn]

Do they sell these deflectors with suction cups to attach to the car?

Mike...

[Arne]

I think you would have to use double sticky tape...